The invention relates to a high-pressure gas discharge lamp comprising
a quartz glass lamp vessel which is saled in a vacuum-tight manner, encloses a discharge space having a major axis and has an ionizable filling containing mercury, rare gas and rare earth halide, PA1 rod-shaped electrodes arranged opposite to each other along the major axis of the discharge space and having respective tips, the tips having a relative distance d, PA1 current supply conductors passed to the exterior through the wall of the lamp vessel opposite to each other and connected to a respective electrode, PA1 the distance d between the tips of the electrodes having a given ratio d/D to the largest diameter D of the discharge space transverse to its major axis, PA1 which lamp consumes during operation a power of at least 250 W. PA1 the discharge space is ovoidal and the distance d between the tips of electrodes lies between 15 and 30 mm, PA1 the electrodes each have near their tips a wrapping of heat-resistant metal wire, which wrapping has a first layer of turns and a second layer of turns around the first layer and leaves free the tip of the electrode, PA1 the ratio d/D lies between 0.75 and 1.25, PA1 the numerical ratio between bromine atoms and iodine atoms in the gas filling Br/I lies between 1.5 and 4, PA1 the lamp consumes during operation at nominal voltage a power lying between 1600 and 2000W. PA1 a concave rotation-symmetrical reflector having an optical axis, an apex at which the optical axis intersects the reflector and an optical centre on the optical axis, which reflector has openings adapted to receive the ends of the lamp vessel of the high-pressure discharge lamp, PA1 the high-pressure discharge lamp transverse to the optical axis of the reflector with the optical centre between the tips of its electrodes, PA1 a screen in the space surrounded by the reflector, which screen is mainly parallel to the electrodes of the lamp and is located at a distance from the optical axis of the reflector at the side of the lamp remote from the apex of the reflector.
The invention further relates to a luminaire provided with such a lamp. Such a lamp is known from GB Patent Specification 1,378,188.
The known lamp is intended inter alia to be used for the illumination of vast sports grounds. The lamp has an elongate tubular lamp vessel of quartz glass, in the proximity of whose ends heat-resistant electrodes are arranged. The distance d between electrodes is larger than and preferably a multiple of the diameter D of the discharge space. In a lamp of 3.5 kW, d is 155 mm and D is 31 mm so that d/D=5. In a lamp of 250W, d is 25 mm and D is 14 mm so that d/D still has the comparatively high value of 1.8. The consumed power per mm distance between the electrodes is very low, i.e. 23 and 10 W/mm, respectively.
In this known lamp, a low power already leads to a considerable electrode gap, while a high power leads to a very large electrode gap and hence to a considerable and a very large discharge arc, respectively. With increasing size of the discharge arc, the size of a luminaire required to concentrate the generated light strongly increases. The quality of the light beam deteriorates.
A disadvantage of sports ground illumination equipments is that they produce a large quantity of scattered light, as a result of which the environment is also illuminated over a large width in a disturbing manner. Besides this disturbing scattered light and the low efficiency associated therewith, the known lamp with a very voluminous discharge arc in respect of the consumed power has the disadvantage that the luminaire, which is consequently necessarily large, requires due to wind-sail and weight a heavy post and hence a heavy foundation. The discharge arc is then voluminous not only because it is long, but also because in horizontal operation (in operation with a straight line through the electrodes in a horizontal plane) it will bulge in upward direction.
Other disadvantages of the known lamp are a strong decrease of the light output due to the phenomenon that the lamp vessel becomes less transparent due to blackening and attack of the quartz, and a comparatively short life of a few hundreds of hours as a result of electrodes breaking off.